**5. Discussion**

#### *5.1. Advantages of the Method and Comparison of Research Results*

Compared with existing research about PLES [55–57], this study conducted research by constructing a fine-grained geographic grid and introducing the coupling coordination degree model of the land the PLEF and combined the coupling and coordination degree of the PLEF between every two of the PLEF for analysis. This overcomes the limitation of the overall coupling and coordination between the administrative unit scale and the single measurement of the PLEF [22]. The research results could provide a valuable reference for clarifying the direction of territorial spatial optimization and enriching the theoretical meaning of the PLES. Moreover, compared with other land use optimization methods [35–38], the evaluation method of PLEF of land was constructed from the perspective of PLES and LUFs in this paper, and the coupling coordination degree model was used to explore the interaction among production, living and ecological functions in the land system. On this basis, we further proposed a space optimization scheme. It has reference value for promoting the application of PLES in the optimization of land use. In addition, the subjective grading method adopted in this article has already referred to the opinions of experienced experts in this field, which to a certain extent can reflect the differences in the strengths and weaknesses of the PLEF of land in the study area.

In addition, it's worth discussing that the research results show that the territorial space of Jiangjin District can be identified as four categories of space: the PLEBS, the PLS, the PES and the ES. Compared with previous studies on the identification of the PLES [21,22], the research results lacked a single production space and a single living space. The reason is that people's production and life are inseparable, and it is difficult to clearly delineate the boundary between the two categories of a single space in terms of spatial expression. In the identification results, the single production space and the living space are relatively small, so they are merged into the PLS.

Moreover, in 2014, the National Development and Reform Commission of China issued "the Thirteenth Five-Year City and County Economic and Social Development Planning Reform and Innovation Guidance Opinions", which required that the proportion of ecological space in key ecological function areas, cities and counties be higher than 50%. In the research results of this study, the ES only accounts for 48.72%. The reason for this is that the PLEBS and the PES both had ecological functions. Therefore, the identification result of this study was in line with the policy requirements, so the research results have reference value.

#### *5.2. Implications for Spatial Management*

In recent years, territorial space governance has gradually become the focus of research on human-environment relationship coordination and sustainable development [58]. The upper reaches of the Yangtze River is an important ecological barrier in the western region of China and the most critical area for the ecological environment protection in the Yangtze River Basin. With the rapid development of industrialization and urbanization, the interaction between human and environment in the upper reaches of the Yangtze River has been strengthened, which has produced a series of ecological problems such as water pollution and ecological destruction [12]. Therefore, it is urgent to strengthen regional territorial space governance and strengthen the construction of ecological barrier in the upper reaches of the Yangtze River. Combined with the results of this study, we suggest that the spatial management strategy in the ecological barrier area of the upper Yangtze River should focus on the following two aspects:

(1) At the macro level, the ecological barrier area in the upper reaches of the Yangtze River should adhere to the principle of ecological priority and green development, a more coordinated planning for space utilization and protection should be formulated. Moreover, the main function zoning and the three zones and three lines should be implemented. In addition, the use of PLEL should be strictly controlled to promote the coordinated development of PLES.

(2) At the micro level, more reasonable optimization measures should be taken in view of the main problems facing the management of PLES. In the territorial space with ecological function as the main body, ecological protection and restoration should be further strengthened, and ecological tourism and ecological industry could be reasonably developed under the premise of ecological protection. In addition, on the basis of strengthening the dominant functions, the territorial space with complex functions will enhance the weak functions and promote the coordinated development of the three functions. For example, in the production-living complex space, public service facilities should be improved to strengthen the leading function of production-living. At the same time, the control of production and living pollution in both urban and rural areas should be strengthened. Moreover, it can increase the ecological land area to improve the ecological function.
